Rheological properties of polyethylenes modified with dicumyl peroxide

Low- and high-density polyethylenes were modified using dicumyl peroxide. The modified polyethylenes were subjected to steady and oscillatory shearing flows. The viscous and elastic properties determined with a cone-and-plate rheometer are correlated to the molecular weight of the materials, which was determined with the aid of size exclusion chromatographic analysis. It was found that both the melt viscosity and elasticity increase with the amount of dicumyl peroxide used, and shear-thinning behavior becomes more intense. The Bueche master curve of viscosity is constructed by using a relaxation time based on the weight-average molecular weight for both virgin and modified resins.     

Thermal memory of polyethylenes analyzed by temperature modulated differential scanning calorimetry

Temperature modulated differential scanning calorimetry (TMDSC) was employed to study the melting and crystallization behavior of various polyethylenes (PEs). Samples of high density PE (HDPE), low density PE (LDPE), linear low density PE (LLDPE), and very low density PE (VLDPE) with different crystal structures and morphologies were prepared by various thermal treatments (isothermal crystallization and slow, fast, and dynamic cooling). The reversing and nonreversing contributions, measured on the experimental time scale, were varied, depending on the crystal stability. A relatively large reversing melt contribution occurs for unstable crystals formed by fast cooling compared to those from slow cooling treatments. All samples of highly branched LDPE, LLDPE, and VLDPE showed a broad exotherm before the main melting peak in the nonreversing curve, suggesting crystallization and annealing of crystals to more stable forms. Other samples of HDPE, except when cooled quickly, did not show any significant crystallization and annealing before melting. The crystallinity indicated that dynamically cooled polymers were much more crystalline, which can be attributed to crystal perfection at the lamellar surface. A reversible melting component was also detected during the quasiisothermal TMDSC measurements. Melting is often accompanied by large irreversible effects, such as crystallization and annealing, where the crystals are not at equilibrium. Such phenomena during a TMDSC scan provide information on the polymer thermal history. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 681–692, 2003     

Rheological study of linear high density polyethylenes modified with organic peroxide

Four high density polyethylenes were modified using different concentrations of an organic peroxide in order to change their molecular structure. The effects of the presence of vinyl groups in the original polymer molecules and of the peroxide concentration used in the modification process were analyzed. All the concentrations of peroxide used in this study were below the critical concentration that produces a macroscopic molecular network. The weight-average molecular weight of all the polyethylenes augments and the molecular weight distribution gets wider as the concentration of peroxide increases. These results support the general belief that the chain-linking reactions dominate the modification process. Evidence of the important role played by the vinyl groups is found not only in the change of the width of the chromatograms but also in the position of their maximums. The vinyl-containing polymers display the largest molecular changes for a given peroxide content. The magnitude of the viscous and elastic moduli of the polyethylenes goes up as the concentration of peroxide used increases showing the effect of the generated large molecules. The linear viscoelastic response of the modified polymers is thermo-rheologically complex. This complexity can be associated with the generation of branched molecules. For similar molecular weights and peroxide concentration, the flow activation energy displayed by the polyethylenes with larger concentration of vinyl groups is larger. This result suggest that a much more complex molecular structure is formed in the presence of vinyl groups. The dynamic moduli of the polymers were analyzed using the generalized viscoelastic model. The spectrum of relaxation times was determined for each polymer and analyzed as a function of the peroxide concentration.     

Shear and elongational rheology of polyethylenes with different molecular characteristics. II. Elongational rheology

The elongational viscosities of polyethylenes with different molecular characteristics were measured at different Hencky strains and temperatures with a capillary rheometer by the replacement of the capillary cylindrical die with a hyperbolic converging die. The hyperbolic shape of the die established a purely elongational flow field at a constant elongational strain rate throughout the die. The effects of molecular characteristics such as the molecular weight, molecular weight distribution, and long‐chain branching and processing conditions such as the temperature and Hencky strain on the elongational rheology of the polyethylene samples were studied. Good master curves were generated for temperature and Hencky strain shifting and simultaneous shifting with respect to both the temperature and Hencky strain. Both the molecular weight distribution and long‐chain branching seemed to promote strain rate thinning and reduce the elongational viscosity. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1184–1194, 2007     

Shear and elongational rheology of polyethylenes with different molecular characteristics. I. Shear rheology

Four metallocene polyethylenes (PE), one conventional low density polyethylene (LDPE), and one conventional linear low density polyethylene (LLDPE) were characterized in terms of their complex viscosity, storage and loss moduli, and phase angle at different temperatures. The effects of molecular weight, breadth of molecular weight distribution, and long‐chain branching (LCB) on the shear rheological properties of PEs are studied. For the sparsely long‐chain branched metallocene PEs, LCB increases the zero‐shear viscosity. The onsets of shear thinning are shifted to lower shear rates. There is also a plateau in the phase angle, δ, for these materials. Master curves for the complex viscosity and dynamic moduli were generated for all PE samples. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Thermal fractionation and identification of low-density polyethylenes

In the present publication, a method to identify and distinguish between different types of low-density polyethylene based on thermal fractionation and FTIR analysis is described. It was shown that during thermal fractionation four to seven endothermic peaks are obtained as opposed to one or two peaks obtained during regular differential scanning calorimetric analysis. The ration between band heights at 1368 and 1376 cm⁻¹ was found to represent the length of the side chain (branch). © 1996 John Wiley & Sons, Inc.     

Chemical modifications and characteristic changes in bacterial cellulose treated with different media

Bacterial cellulose membranes have attracted a great deal of attention as novel biomedical materials. In this paper, bacterial cellulose was treated with acidic, alkaline, and redox solutions to investigate the subsequent changes in the characteristics of the cellulose. The chemical structure, crystalline state, water-holding capacity, and micromorphology of each modified BC were characterized by FTIR, SEM, and XRD. After these treatments, the intermolecular or intramolecular hydrogen bonds of the BC were broken and the water bound to the BC was released from the BC hydrogel. During these processes, the crystallinity and surface morphology of the BC were also modified. Meanwhile, the crystalline form of the BC changed from cellulose I to II in alkaline medium. In particular, the BC nanofiber hydrogel broke into floccules when treated with highly concentrated NaOH solution at a temperature of ?5 °C, but these floccules congregated into a bulk state again after removing the NaOH.     

Determination of the molecular characteristics of commercial polyethylenes with different architectures and the relation with the melt flow index

The molecular weight distribution curves of several commercial polyethylene samples were evaluated by high‐temperature gel permeation chromatography with two detectors (a refractive‐index detector and a viscometer) to determine the molecular sizes and architectures (branching). The polymer samples included high‐ and low‐density polyethylenes with different molecular weight distributions (wide, medium, unimodal, and bimodal) from nine producers. The results were tested against the melt flow index and zero‐shear melt viscosity to find correlations. The data for high‐density polyethylene correlated well with the molecular weight, whereas the data for low‐density polyethylene did not correlate. However, when the weight‐average molecular weight was corrected by the branching parameter and a factor form, all the polyethylene samples fit a single equation. These results indicate that the melt flow index is dependent not only on the molecular weight but also on the molecular shape, including branching. The relation accounted for samples of different resin producers, molecular weights (65,000–638,000), and polydispersities (2.9–20). The use of the branching parameter for the correction of the molecular weight allowed the correlation of these parameters despite differences in the technologies, molecular weights, and molecular architectures. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1572–1578, 2007     

Diffusional characteristics of coextruded linear low-density polyethylenes prepared from different conditions of processing

The permeability and diffusivity of oxygen, carbon dioxide, nitrogen, and helium have been obtained for a range of linear low density polyethylene (LLDPE) films prepared from the same raw materials but with different processing conditions. The measurements were carried out by means of a permeation technique over the temperature interval where the α-relaxation processes were observed in earlier studies. The temperature dependence of the permeability and diffusion coefficients of gases shows 2 well-differentiated regions in all films. The break temperature of these regions is approximately located at the same temperature as the α-relaxation takes place. Both the permeability and their temperature dependence do not show a noticeable influence on the processing conditions. The effect of processing conditions on the diffusivity seems to be more complex. Differences are observed for different films in the diffusion coefficients, in the case of oxygen, and in their change with the temperature, which is particularly marked in the case of carbon dioxide. Fujita's free volume model has been applied to diffusivity data in order to study the influence of films microstructure in gas permeation properties through them. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 23–37, 1998     

Tensile mechanical behavior of linear high‐density polyethylenes modified with organic peroxide

The molecular structure of several high‐density polyethylenes of different molecular weights and vinyl contents was modified without altering their thermoplastic character using an organic peroxide. Chain linking was the main chemical event that occurred during the modification process. Samples of these polymers were crystallized from the melt, generating materials with different morphologies. Two crystallization procedures were followed: slow cooling and quenching. The density and crystallinity of the polymers were found to be slightly dependent on the molecular structures generated by the modification process. Tensile tests were performed at room temperature to evaluate the mechanical behavior of the polymers. The mechanical response of some of the slowly cooled samples changed from brittle to ductile when increasing concentrations of peroxide were added to the formulation. All the quenched samples displayed ductile behavior. The elastic modulus and yield stress were found to increase linearly with the crystallinity of the polymers independently of the molecular structure generated by the modification process. The molecular weight of the modified polymers appears to be the main structural property that influences the draw ratio after break and the ultimate tensile stress of the samples. The draw ratio diminishes, while the ultimate tensile stress increases with the molecular weight of the polymers, irrespective of the evolution of other molecular parameters.     

Crystallization behavior of hyperbranched polyethylenes with different degree of branching

Hyperbranched polyethylenes (HBPEs) with different degree of branching (DB) and similar M_n are used to investigate the effect of branch structure on their crystallization behaviors. The crystal structure, isothermal, and non‐isothermal crystallization kinetics of HBPEs are studied by X‐ray diffraction and differential scanning calorimetry. The isothermal crystallization process is analyzed by the Avrami equation while the non‐isothermal crystallization process is analyzed through the Ozawa and Mo methods. The XRD results indicate that the crystallization ability of HBPEs is weakened with the introduction of branch structure, i.e., the crystallinity of HBPEs decreases with the increase of DB, and even tends to zero. The kinetics results of isothermal and non‐isothermal crystallization verify the peculiar effects of DB on the crystallization process of HBPEs. In detail, a little of branch structure can accelerate the crystallization process of HBPEs, however a large number of branch can inhibit it. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44127.     

Solution-induced changes in the properties of polyethylenes

The chain entanglement states in high density, linear low density, and low density polyethylenes (HDPE, LLDPE, and LDPE) have been modified by recovering the polymers from solutions in trichlorobenzene (TCB) and p-xylene. In the thermodynamically good solvent, TCB, the entanglement density is assumed to be sharply reduced, a condition which is carried over to the corresponding solids. These display transient, but large increments in tensile moduli, slight changes in stress at rupture, and decreases in dynamic mechanical parameters and in elongation at rupture. Scanning calorimetry also shows these solids to have reduced crystallinity. Much smaller property modifications are noted in corresponding samples recovered from p-xylene. This liquid is a poorer solvent, particularly for HDPE and LLDPE. The results indicate that property modifications due to deliberate changes in the entanglement states of the polymers are a general phenomenon in the polyethylenes, and the magnitude of property changes depends on parameters of the molecular weight distribution. Chain branching does not seem to be a leading factor in the sensitivity of properties to modifications in the entanglement states. The property modifications produced by the present solution treatments are viewed as guides to the magnitude and duration of shear refining effects to be expected in HDPE, LLDPE, and LDPE polymers.     

Decomposition of hydrogen peroxide in the presence of activated carbons with different characteristics

BACKGROUND: Catalytic ozonation promoted by activated carbon is a promising advanced oxidation process used in water treatment. Hydrogen peroxide generated as a by‐product from the reaction of ozone with some surface groups on the activated carbon or from the oxidation of some organic compounds present in the water being treated seems to play a key role in the catalytic ozonation process. Hydrogen peroxide decomposition promoted by two granular activated carbons (GAC) of different characteristics (Hydraffin P110 and Chemviron SSP‐4) has been studied in a batch reactor. The operating variables investigated were the stirring speed, temperature, pH and particle size. Also, the influence of metals on the GAC surface, that can catalyze hydrogen peroxide decomposition, was observed. RESULTS: Chemviron SSP‐4 showed a higher activity to decompose hydrogen peroxide than HydraffinP110 (70 and 50% of hydrogen peroxide removed after 2 h process, respectively). Regardless of the activated carbon used, hydrogen peroxide decomposition was clearly controlled by the mass transfer, although temperature and pH conditions exerted a remarkable influence on the process. Catalytic ozonation in the presence of activated carbon and hydrogen peroxide greatly improved the mineralization of oxalic acid (a very recalcitrant target compound). About 70% TOC (total organic carbon) depletion was observed after 1 h reaction in this combined system, much higher than the mineralization achieved by the single processes used. CONCLUSIONS: Of the two activated carbons studied, Chemviron SSP‐4 with an acidic nature presented a higher activity to decompose hydrogen peroxide. However the influence of the operating variables was quite similar in both cases. Experiments carried out in the presence of tert‐butanol confirmed the appearance of radical species. A kinetic study indicated that the process was controlled by the internal mass transfer and the chemical reaction on the surface of the activated carbon. The catalytic activity of hydrogen peroxide in oxalic acid ozonation promoted by activated carbon (O₃/AC/H₂O₂) was also studied. The results revealed the synergetic activity of the system O₃/AC/H₂O₂ to remove oxalic acid. Copyright © 2010 Society of Chemical Industry     

Solubility and diffusivity of cyclohexane in two different polyethylenes

The solubility and diffusivity of cyclohexane in a very low density polyethylene (VLDPE) have been measured over a range of temperatures and concentrations. These data provide an interesting contrast to similar data for high density polyethylene. Above the melt temperatures, the solubilities for both polyethylenes follow the same correlation as a function of the activity. Below the melt temperature, however, a correction is needed to correlate the solubility that accounts for the elasticity effects caused by the polymer chains between the crystals. The capabilities of two models that can predict the solubility and the free‐volume model for the correlation of the diffusivity have been analyzed. Unlike in previous studies, the diffusivity in the VLDPE does not appear to be influenced significantly by the presence of crystals. POLYM. ENG. SCI., 55:688–692, 2015. © 2014 Society of Plastics Engineers     

Study of kinematics and dynamics of film blowing of different polyethylenes

An extensive experimental study of the effects of material characteristics and processing parameters on the kinematics and dynamics of film blowing is presented. Three polyethylene resins, a high-density polyethylene (HDPE), a low-density polyethylene (LDPE), and a linear low-density polyethylene (LLDPE) were investigated. The convergent flow analysis of Cogswell was used to characterize the elongational flow behavior of the polymers. Strain rates and pressure inside the bubble (P_i) have been determined over a wide range of film blowing conditions. Moreover, on-line bubble temperature and birefringence measurements have been carried out along the length of the bubble. The experimental results reveal that the three polymers display different behaviors. The LLDPE requires the highest P_i value and the LDPE, the lowest. Consistent with this, the LLDPE shows the lowest in-plane birefringence and the LDPE, the highest. Interactions between various process parameters affecting the P_i value are characterized. Bubble instability is correlated to the apparent uniaxial elongational viscosity and P_i. The most stable polymer (LDPE) has the highest elongational viscosity and requires the lowest P_i. Stresses have been calculated with the help of the birefringence and P_i data. The stress and strain rate data were used to calculate an apparent nonuniform biaxial elongational viscosity of the melts, but could not be correlated through any simple constitutive equation.     

不同晶型聚丙烯的光老化行为

采用熔融挤出法分别制备了α-成核聚丙烯(α-PP)和β-成核聚丙烯(β-PP),通过傅里叶变换红外光谱和立体显微镜研究了氙灯辐照对纯聚丙烯(PP)、α-PP和β-PP微观结构和表面形貌的影响。在相同老化条件下,α-PP最易老化,PP次之,β-PP最稳定;试样老化后表面裂纹形态不同,α-PP和PP表面呈破裂状无规排布的细长裂纹,而β-PP表面的裂纹呈圆弧状有规律的间隔排布。     

Thermal and oxidative effects in polyethylenes above 200°C

There is considerable literature on the behavior of polyethylene below 200°C and above 400°C. This paper presents results on both high and low density polyethylene between 200⁰ and 400°C, a range of practical import for various fabrication and processing steps. Practical exposure times may be short and the experimental procedures were designed to give meaningful results for times as low as 1 min. The principal effects are chain-session and oxidation. Session was isolated by applying an atmosphere of pure nitrogen and proved uite shlow at 300°C but rapid at 400°C. The course of scission and produces discoloration, odor, and rising carbonyl content. A brief discussion is given of the role of diffusion in oxidation.     

Thermal decomposition of uranium peroxide hydrates

The thermal decomposition of uranium peroxide hydrates, UO₄·4H₂O and UO₄·2H₂O, has been investigated by thermal analyses, X-ray diffraction study and infrared spectroscopy. As a result, it is concluded that the thermal decomposition of uranium peroxide hydrates proceeds in the following sequence:     

Comparing cracking time and structure changes of different high‐density polyethylenes during stress and photo‐oxidative aging

This article describes the structure changes of high‐density polyethylene (HDPE) during stress and photo‐oxidative aging experiments, and the relationship between different materials and cracking time. The three most representative grades of HDPE are 9070, TR480, and 2480NT. The average molecular weight, the comonomer type, and content of materials were measured by high‐temperature gel permeation chromatography, ¹³C nuclear magnetic resonance (NMR) spectroscopy, and successive self‐nucleation and annealing technique. Moreover, tensile testing was done to distinguish different toughness of materials. The samples were exposed to 5 MPa stress and ultraviolet irradiation in an aging oven, and observed at time intervals. The changes in structure were characterized by metallurgical microscopy, differential scanning calorimetry, attenuated total reflection‐Fourier transform infrared spectroscopy, X‐ray diffraction, and gel content measurements. With increasing time, the crystallinity increased, whereas melting point and oxygen induction times decreased. Meanwhile, the carbonyl index values and gel content reached about 10% until the samples were cracked. The results manifested that the resistance to cracking of the different HDPEs followed the order: 2480NT > TR480 > 9070. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40904.     

Thermal decomposition of uranium peroxide hydrates

The thermal decomposition of uranium peroxide hydrates, UO₄·4H₂O and UO₄·2H₂O, has been investigated by thermal analyses, X-ray diffraction study and infrared spectroscopy. As a result, it is concluded that the thermal decomposition of uranium peroxide hydrates proceeds in the following sequence: where 3 ≤ x ≤ 3.5.